In Rev. 9:4 we read that it was commanded to locusts "that they should not hurt the grass of the earth, neither any green thing, neither any tree", but just nine verses earlier, in Rev. 8:7, we are told that when the first angel sounded "all the green grass was burnt up". How is it so?
Context is the Key
In order to properly interpret this passage, one has to determine the context in which it was written. There are varieties of these two, but quite simply: Is this passage to be interpreted Literally(the grammatical and historical meanings of the words used) or Figuratively(representative of another reality).
The context can be determined by the rest of the verse (4),
"... neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads."
Since the "seal of God on the forehead" is not a physical seal, but rather a spiritual one,
In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that Holy Spirit of promise,(Eph. 1:13)
And grieve not the Holy Spirit of God, whereby ye are sealed unto the day of redemption.
The "sealing" takes place when a believer, by faith, receives Christ and the Holy Spirit "seals" or makes binding his commitment, by indwelling in the believer. Since the 'locust', or demonic horde unleashed on the earth apparently recognize this seal, it's pretty obvious they are not sent to physically harm the earth, but 'spiritually harm' the unbelievers. The rest of the imagery of the locust does not describe a locust you would find in your back yard, rather they have 1)Crowns of gold, 2) Faces of men, 3) Hair of women, 4) Teeth of Lions, 5) Breastplates of Iron, 6) Tails of Scorpions, of which their sting doesn't kill, but makes one desirous of death. To use the words of J.N. Darby, founder of Dispensationalism, it would do "violence to the text" to literally interpret an obviously figurative passage: there is no creature in nature that one can identify as "Literally" describing what is being identified, therefore we must determine what is being "figuratively" described. It's features tell us what it is and what it does. I won't go into detail about what the locust represents, since the OP hasn't asked for its meaning but that is why the 'grass' isn't consumed, the creature being described is not a 'locust' in the traditional meaning of the word.
The "grass" here is not the issue, as we are not talking about 'physical insects' and the context is not physical destruction. The context is about the spiritual torment, from which this horde from Hell is released upon the earth to exact on those who are not "sealed" by the Holy Spirit. The purpose in saying, "...do not harm the grass of the earth, neither any green thing, neither any tree" is to emphasize this truth; regardless of whether the physical earth was harmed by previous judgments.
has it crossed our mind that the green trees and grass are punished in the first trumpet, and then protected in the fifth trumpet....this is not literal trees and grass....trumpet 5 brings it out clearly that the green trees and grass are representing Gods people who are in good favour with God(with the seal of God)...trumpet 1 shows those who were once Gods people who have forsaken the covenant being punished (mathew 7:19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20So then, by their fruit you will recognize them. 21Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of My Father in heaven.…) the imagery can be seen in Ezekiel 20:46-48.....also the parable of the barren fig tree illustrates what happens when God s people ignore the privileges they are given and produce no fruit ....the cursed fig tree by Christ was an acted parable ,indicating that the jewish nation was represented by the fig tree....green leaves but no fruit!and the axe was already on the roots of that tree....so the first trumpet must point to NOT BURNING OF LITERAL TREES but to a judgement on those who reject Gods covenant who have been given privilege(the number one third in revelation is ALWAYS ASSOCIATED WITH THOSE FACING JUDGEMENTS,those in rebellion to God s word...)....the second trumpet shows a kingdom collapsing...(mountain burning being thrown into the sea...jeremiah 51:25)....the third trumpet shows a perversion of God s truth..see Jeremiah 9:13-15)...the fourth trumpet shows deeper apostasy on those who have rejected the truth(dark ages)(compare the church in Revelation 12 with full sun and moon and stars shining while in the 4th trumpet light from Christ and scriptures is BLOCKED)...the fifth trumpet shows demonic hordes sent to punish the rejecters of the truth(please note the sun is completely blocked in trumpet 5...total darkness and Satan reigns for five months like was the flood time in Noah s time) this is a sad time when God withdraws his light and humans plunge into darkness unprotected from God s mercy.....we are taken in imagery to the protection given to Noah s family and while others WERE SWEPT AWAY...in this 5th trumpet terrible conditions must exist with spiritual torment upon those who have no seal of God...no human agents can identify that seal except spiritual forces hence demonic army is released from Abyss (see luke 8:31) and unleashed on rejecters of the truth...this event happens in a place called spiritually egypt and sodom and gomorrah...you cannot interpret trumpet 5 and 6 without chapter 10 and 11 of revelation its impossible....you ask questions like this ...when was the abyss opened?its answered in revelation 11:7...what event is in view?....there is a deeper meaning in those texts than generalisations made...i have avoided to give specific events to the trumpets to avoid prejudice but i can give you evidence for one of them...the second trumpet points to the fall of pagan Rome....John uses Jeremiah 51:24,25,42 61,62 linguistic references allusions to show the reader that a large kingdom as was like babylon is seen collapsing.....thats pagan Rome falling after barbarian invasions....for first trumpet check who got punished in AD70 who was God s favourite tree but who became fierce rejectors of the truth...for third trumpet check who was Balaam in the church of pegamum....these seven churches in asia Minor were symbolic of the church s exercise throughout the christian era so during the Pergamum church phase the christian church began to seek favour with state power for favours (like Balaam ) and church were happy to allow perversion of their originally clean teachings(hence wormwood)...by the 4 th church we have jezebel in church and the church plunged into apostasy... and there was famine of the WORD IN THE LAND(darkness)...i could go on !